1. Field of the Invention
The present invention is generally related to a method for determining the amount of liquid in a reservoir as a function of a linear measurement provided by a depth determining device and, more particularly, to a method for automatically calibrating the relationship between the liquid depth and the volume of the liquid remaining in the reservoir notwithstanding the potentially non uniform shape of the liquid reservoir.
2. Description of the Prior Art
Many different types of fuel tanks and fuel depth sensors are known to those skilled in the art. Most fuel tanks are provided with a sensor that determines, either directly or indirectly, the amount of liquid fuel remaining in the fuel tank. Most typically, a float mechanism is used to measure the depth of the liquid in the reservoir and that measured depth is used to determine or represent the amount of liquid remaining in the reservoir. These types of devices are most commonly used in fuel tanks, such as the fuel tanks of automobiles, boats, and other vehicles.
When a fuel tank, or other liquid reservoir, has an irregular shape, the relationship between the depth of the fluid within the reservoir and the volume of fluid remaining in the reservoir is not always linear. In other words, each decrease in the depth, by a particular incremental magnitude, does not always represent the same change in the liquid volume remaining in the reservoir. Most fuel tanks are not purely parallelepipeds, with six parallel faces. If the tank is a parallelepiped, the volume remaining in the tank generally has a linear relationship with the depth of the liquid actually remaining in the tank. However, this is rarely the case in actual practice since most fuel tanks have irregular shapes, particularly when used in marine vessels.
Many different approaches have been used to address the accuracy of fuel measurement in these types of situations.
U.S. Pat. No. 5,485,740, which issued to Lippmann et al on Jan. 23, 1996, describes a method of calibration for gauging fuel in an automotive tank. The maximum fuel level is determined in a fuel tank of undetermined size using a fuel level sender which is referenced to the tank bottom. A minimum full value is selected for a given type of tank and when the sender signal goes below a percentage of that value and subsequently goes above the minimum full value, a fueling event is recognized and the current fuel sender value is adopted as the maximum full value. Where a heavily filtered signal is used to minimize the effect of fuel slosh, the maximum full value is subject to increasing to higher values occurring during a short period after fueling to allow recovery of the filtered signal. An empty value is calibrated before the vehicle is initially fueled. A maximum empty level is initially set and if a lower level is measured when the ignition is turned on, the lower level is set as the empty value. This calibration is terminated when the tank is fueled above the minimum full value.
U.S. Pat. No. 5,752,409, which issued to Lippmann et al on May 19, 1998, describes a method of accurately gauging fuel in an automotive tank. This patent is generally related to U.S. Pat. No. 5,485,740.
U.S. Pat. No. 6,289,728, which issued to Wilkins on Sep. 18, 2001, describes an apparatus and method for determining the amount of liquid contained in a storage tank. A tube has a closed bottom end and is disposed within the tank in a substantial vertical orientation with the closed end of the tube immersed in the liquid and the open end of the tube protruding from the top of the tank. An elongate member has a longitudinal edge defining a varying profile corresponding to the predetermined relationship between the liquid level and the barometric content. This member is slidably received within the tube. A float is slidable vertically along the exterior of the tube and is magnetically coupled to the elongate member such that as the float rises or falls in response to a change in liquid level, the elongate member will be vertically displaced within the tube. A measuring device position proximate the open end of the tube measures the distance between the longitudinal edge of the elongate member and a reference plane aligned substantially parallel to the longitudinal axis of the elongate member, the distance being directly proportional to the volumetric content of liquid contained in the tank.
U.S. Pat. No. 4,731,730, which issued to Hedrick et al on Mar. 15, 1988, describes a universal fuel quantity indicator apparatus. The apparatus includes a digital fuel quantity indicator having an internal microprocessor control unit and a removably connectable digital calibration trim interface which enables the fuel quantity indicator to be reconfigured for different fuel tanks by varying selectable fuel tank parameters, such as constants associated with a fuel quantity determination based on calculations of capacitance of a tank capacitor array and a compensator capacitor. The interface includes up/down steering control buttons which control selection and storage and of the selected parameters, a mode selection jumper which selects between a RUN mode a CALIBRATE mode for the indicator, and an alternate static memory which stores the configured parameters for loading to the indicator internal static memory associated with the microprocessor in the CALIBRATE mode to configure the indicator for a specific fuel tank.
U.S. Pat. No. 4,724,705, which issued to Harris on Feb. 16, 1988, describes a fuel gauge. The apparatus is provided for determining the quantity of fuel in a fuel tank having a contoured interior wall. The apparatus includes a buoyant member for floating on the top surface of fuel in the fuel tank, a conductor supported on the buoyant member to float therewith, and a pattern of resistance in electrical contact with the floating conductor. The pattern of resistance is selected to correspond to the contour of the interior wall. Thus, the resistance pattern is coded to represent the volume profile of the fuel tank. A voltage is applied across the conductor and the pattern of resistance. The floating conductor and the pattern of resistance cooperate to provide a value of resistance corresponding to the depth of fuel in the fuel tank at the measurement location. An indicator interprets that value of resistance to determine the instantaneous quantity of fuel in the fuel tank.
U.S. Pat. No. 4,358,947, which issued to Greene et al on Nov. 16, 1982, describes a method and apparatus for volumetric calibration of liquid flow sensor output signals. A liquid flow sensor produces output signals which are monitored over a known volume of flowing liquid. That monitored signal value is scaled according to a predetermined volume of liquid to provide a correction factor, whereby the output signal of the flow sensor is continually corrected to provide an expected number of flow sensor pulses per unit volume.
U.S. Pat. No. 5,315,867, which issued to Hartel et al on May 31, 1994, describes an apparatus for measuring the fraction of liquid fuel in a fuel tank. The quantity of fuel in a fuel tank is measured by determining the displacement of a membrane and pressure values from a sensor. The displacement values of the membrane are representative of volume changes in a gas chamber in the tank while the pressure values in the sensor are representative of the pressure of the gas chamber. With these values, the fuel content in the gas tank can be determined from the general equation of state of an ideal gas. In order to displace the membrane, an electric motor drives a step down transmission which in turn drives a spindle mechanism which is coupled to the membrane.
U.S. Pat. No. 4,840,064, which issued to Fudim on Jun. 20, 1989, describes a liquid volume monitoring apparatus and method. The apparatus is disclosed for determining the volume of liquid in a container which is being provided with an in flow and an out flow of liquid such that there normally remains an ullage volume in the container filled with gas. A timer actuated valve is provided for interrupting one of the in flow and the out flow of liquid. A pressure sensor detects the pressure of the gas in the ullage volume of the container during the interruption of the one of the in flow and out flow of the liquid and a microprocessor calculates the volume of liquid in the container from the detected change in pressure of the gas during the interruption. The apparatus and associated method are particularly advantageous for determining the volume of liquid in a liquid reservoir of an integrated drive generator of an aircraft engine since the determination of the liquid volume can be accomplished with the reservoir of irregular shape in various attitudes, at various temperatures and under conditions of acceleration and minimal additional weight being added to the aircraft.
U.S. Pat. No. 4,884,444, which issued to Tuckey on Dec. 5, 1989, describes a motor-driven material level indicator. The apparatus is intended for indicating level of fluent materials, such as gasoline in an automobile tank, which comprises an impeller extending vertically through at least a major portion of the vessel in physical contact with material in the vessel. An electric motor is coupled to the impeller and connected to a source of electrical energy for rotating the impeller against drag imparted thereto by the material. Material level is indicated as a continuous function of load applied to the motor by the rotating impeller.
The patents described above are hereby expressly incorporated by reference in the description of the present invention.
Most fuel tank measurement devices measure a linear dimension, such as the depth of fuel remaining in the fuel tank, and then use that linear dimension as a representation of the volume of fuel remaining in the fuel tank. The volume of liquid remaining in a liquid reservoir, such as the fuel tank described above, is typically measured in this way. However, many types of fuel tanks are not uniform in volume at all of the possible magnitudes of the incremental changes in linear depth measurement. As a result, the depth of fuel remaining in a fuel tank is not always a reliable indicator of the actual volume of fuel remaining in the fuel tank. It would therefore be significantly beneficial if a measuring system could be provided which is self calibrating and, through normal use of the fuel tank, determines an accurate relationship between the linear depth measurement and the volume remaining in the fuel tank for all of the potential depth measurements between a minimum magnitude, when the tank is empty, and a maximum magnitude, when the tank is full. It would also be beneficial if the system could be self calibrating without the need for prior knowledge of the tank""s shape or linear dimensions.
A method for determining the amount of liquid in a reservoir, in accordance with a preferred embodiment of the present invention, comprises the steps of measuring a depth of the liquid in the reservoir during a period of time when the depth of the liquid changes from a first depth magnitude to a second depth magnitude and also monitoring an amount of the fluid which is removed from the reservoir during the period of time when the depth of the liquid changes from the first depth magnitude to the second depth magnitude. The method further comprises the step of determining a first relationship between the difference between the first and second depth magnitudes and the amount of the fluid which is removed from the reservoir during that period of time when the depth of the liquid changes from the first to the second depth magnitude. The method comprises the step of storing the first relationship and using the first relationship to determine an amount of the fluid remaining in the reservoir as a function of a measured depth magnitude which is between the first and second depth magnitudes.
The present invention can further comprise a step of calculating a second relationship between an amount of the fluid removed from the reservoir when the depth of the liquid changes from the second depth magnitude to a minimum depth magnitude. The method of the present invention can further comprise the step of calculating a third relationship between an amount of the fluid removed from the reservoir when the depth of the liquid changes from a maximum depth magnitude to the first depth magnitude. In a preferred embodiment, the second and third relationships are linear.
The steps of the present invention comprise providing a depth measuring device, measuring a change in the depth of the liquid within a reservoir, measuring a quantity of liquid removed from the reservoir contemporaneous with the change in depth from a first depth magnitude to a second depth magnitude. It further comprises the step of determining a quantitative relationship between the change in the depth and the quantity of liquid removed from the reservoir contemporaneous with the change in depth. Also, the present invention comprises the step of storing the quantitative relationship in association with the first and second depth magnitudes.
The method of the present invention can comprise an initial step of providing an estimated relationship between a plurality of magnitudes of liquid remaining in the reservoir and an associated plurality of magnitudes of depths of the liquid. The estimated relationship can be stored in a table within a memory device of a microprocessor. During the performance of the method of the present invention, it can further comprise the step of modifying a portion of the estimated relationship, which is determined as a function of the first and second depth magnitudes, through the use of the quantitative relationship between the change in the depth and the quantity of liquid. It can further comprise the step of changing the estimated relationship to reflect a linear relationship between the change in the depth and the quantity of liquid for magnitudes of the depth between the second depth magnitude and a minimum expected magnitude of the depth magnitude. Additionally, the method of the present invention can further comprise the step of changing the estimated relationship to reflect a linear relationship between the change in the depth and the quantity of liquid for magnitudes of the depth between the first depth magnitude and a maximum expected magnitude of the depth magnitude.
The modified portion of the estimated relationship can be selected as a function of the average value of the first and second depth magnitudes or, alternatively, as a function of the range defined by the values of the first and second depth magnitudes.
In certain applications of the present invention, the second depth magnitude is generally equal to a minimum expected magnitude of the depth magnitude, such as when the reservoir is empty. The first depth magnitude is generally equal to a maximum expected magnitude of the depth magnitude, as when the tank is filled, in a preferred embodiment. The relationship between the change in the depth and the quantity of liquid is a slope calculated by dividing the quantity of liquid by the change in the depth, in certain preferred embodiments of the present invention. As described above, the liquid is typically a fuel, such as gasoline, and the fuel is injected by a fuel injector associated with an internal combustion engine. The quantity of fuel removed from the reservoir, as described above, is measured by accumulating the quantity of fuel injected by the fuel injector during the change in depth from the first depth magnitude to the second depth magnitude.